Representation of all possible interactions between
bases involving at least two H bonds. Some of the non-Watson-Crick
interactions (b, c, g, k, l, m, q, and r) were mentioned by Donohue as
early as 1956. Interactions involving N3 of purines are not considered.
A curved arrow on a covalent bond means that a symmetric configuration
obtained by a 180 deg rotation around thi sbond must be considered.
This simplification has not been used in the particular cases of
Watson-Crick (a and b) and Hoogsteen (e and f) pairing between A and U.
It should be kept in mind that this figure is only schematic and is not
intended to be exact regarding the angles of H bonds. Also, some of the
interactions (e.g., s and t) are illustrated for completeness and may
not ever be observed in real structures.